1. Field of the Invention
The present invention relates to a thermal printer having a cooling fan for cooling a thermal head and a control method of controlling the cooling fan.
2. Description of the Prior Arts
A thermal printer is provided with a thermal head in which plural heating elements are arranged in rows in a main scanning direction. An image is printed on a recording paper by heating the heating elements that are contacted with a surface of the recording paper while feeding the recording paper in a sub-scanning direction. In order to obtain a high quality image, the temperature of the thermal head (hereinafter referred to as the head temperature) needs to be kept appropriately. However, if printing is continuously performed, it is caused to accumulate heat in the thermal head, so that it becomes impossible to print the image of which density is appropriate. In order to prevent the heat accumulation in the thermal head, a heat sink for radiating heat is provided in the thermal head and controlled by a cooling fan.
In a commercial thermal printer, an air amount of the cooling fan is not controlled. Accordingly, the air amount is constant during driving the cooling fan. Therefore, for example, when a white solid image such as a snow scene and a black solid image such as a night scene are printed alternately and continuously, in printing the white solid image, the head temperature becomes low due to a low printing rate (printing density); meanwhile, in printing the black solid image, the head temperature becomes high due to a high printing rate. As a result, the head temperature gradually increases or decreases repeatedly up and down in response to cooling ability of the cooling fan.
The change of the head temperature in one printing period is shown in FIG. 4. A printing image in which a white (blank) solid area and a black solid area are arranged alternately is shown in FIG. 4(a). A state that the cooling fan is continuously driven in printing the black and white solid areas is shown in FIG. 4(b). The change of the head temperature measured by a measurer such as a thermography is shown in FIG. 4(c). The head temperature becomes high when printing the black solid area, while the head temperature becomes low when printing the white solid area. In such printing, since the air amount of the cooling fan is large, the head temperature gradually becomes low during changing a corrugated form. If the air amount is small, the head temperature gradually becomes high during changing the corrugated form. Therefore, density unevenness is created in the same print, and to make matters worse, density difference is generated between prints. In addition, if the printer is placed in a high or low temperature environment, the cooling ability of the cooling fan is changed. Therefore, the change of the head temperature becomes large, so that the density difference is generated from the difference in environments.
In order to solve the above-mentioned problems, in the thermal printer disclosed in Japanese Patent Laid-Open Publication No.H6-255141, the head temperature sensor for measuring the head temperature is provided in the thermal head to control the head temperature by controlling the air amount of the cooling fan based upon the measured temperature information from the sensor. In the thermal printer disclosed in Japanese Patent Laid-Open Publication No.H6-42494, the head temperature is controlled by controlling the air amount of the cooling fan based upon the measured temperature information from the head temperature sensor, applying the fuzzy theory.
However, the head temperature is not enough to be controlled by the above-mentioned methods. If the two images having different printing rates, for example the black and white solid areas, are printed alternately or randomly, the head temperature is fluctuated. For example, when the black and white solid areas are printed alternately as shown in FIG. 5(a), the actual temperature of the thermal head is fluctuated as shown in FIG. 5(b). However, with respect to the fluctuation of the head temperature measured by the head temperature sensor, delay of Δt minutes occurs to the fluctuation of the actual head temperature.
This delay time is caused by a delay in both a heat transmitting system and a measuring system. The delay in the heat transmitting system is attributable to an attachment position of the head temperature sensor, material of the thermal head and a heat sink, and a shape of an air flowing path for leading a cooling air sent from the cooling fan, while the delay in the measuring system is attributable to timing of temperature data acquisition in the head temperature sensor. When the actual head temperature is changed, it takes several seconds to several tens of seconds to detect such temperature change by the head temperature sensor. Accordingly, as shown in FIG. 5(d), the control of the cooling fan is delayed. For example, if the black and white solid areas are printed alternately in the same printing paper, although the actual head temperature is lowered when printing the white solid area, it takes considerable time to reflect in the control of the air amount of the cooling fan after measuring such temperature change by the head temperature sensor. Therefore, when the air amount is controlled, the black solid area has already started to be printed.
When the black solid area is printed, since the air amount is kept low, the actual head temperature becomes high. When the air amount increases after detecting the temperature change by the head temperature sensor, the white solid area has already started to be printed, so that there is a problem that the actual head temperature is rapidly lowered. Accordingly, since the control of the air amount of the cooling fan is delayed according to the kind of image to be printed, the fluctuation of the head temperature is not enough to be controlled, so that the density difference between prints and the density unevenness in the same print are created.